[dune-fem] spmatrix change proposal

[Agnese, Marco]

Hi everybody,
we are writing some wrappers for LDL and SPQR. 
Since the sparse matrix format is the same of UMFPack it would be nice to use the method SparseRowMatrix<T>::setupUMF() to extract the matrix in CCS format. 
This method is now private therefore I ask if it is possible to make it public (and maybe rename extractCCSMatrix or something like that).
Thank you very much,
cheers,
Marco.

PS: I know that an alternative solution is to copy the code in my own wrapper but it leads to useless code duplication and furthermore the class variable clearedRows_ is private.